Semester_2/IntroOS/OSC10e/ch13/LockingExample.java

/**
 * A simple program demonstrating file locking.
 * This program acquires an exclusive lock on the
 * first half of the file and a shared lock on the second half.
 *
 * Usage
 *	java LockingExample <input file>
 *
 * Figure 13.2
 *
 * @author Gagne, Galvin, Silberschatz
 * Operating System Concepts - Tenth Edition
 * Copyright John Wiley & Sons - 2018.
 */

import java.io.*;
import java.nio.channels.*;

public class LockingExample {
	public static final boolean EXCLUSIVE = false;
	public static final boolean SHARED = true;

	public static void main(String args[]) throws IOException {
		if (args.length != 1) {
			System.err.println("Usage: java LockingExample <input file>");
			System.exit(0);
		}

		FileLock sharedLock = null;
		FileLock exclusiveLock = null;

		try {
			RandomAccessFile raf = new RandomAccessFile(args[0], "rw");

			// get the channel for the file
			FileChannel channel = raf.getChannel();

			System.out.println("trying to acquire lock ...");
			// this locks the first half of the file - exclusive
			exclusiveLock = channel.lock(0, raf.length() / 2, EXCLUSIVE);
			System.out.println("lock acquired ...");

			/**
			 * Now modify the data . . .
			 */

			try {
				// sleep for 10 seconds
				Thread.sleep(10000);
			} catch (InterruptedException ie) {
			}

			// release the lock
			exclusiveLock.release();
			System.out.println("lock released ...");

			// this locks the second half of the file - shared
			sharedLock = channel.lock(raf.length() / 2 + 1, raf.length(), SHARED);

			/**
			 * Now read the data . . .
			 */

			// release the lock
			exclusiveLock.release();
		} catch (java.io.IOException ioe) {
			System.err.println(ioe);
		} finally {
			if (exclusiveLock != null)
				exclusiveLock.release();
			if (sharedLock != null)
				sharedLock.release();
		}
	}
}
